You are trying to create an ultra-wide video from a source that is not ultra-wide. The other forum members and I took it for granted that you would do some type of cropping or scaling on the source video to make it fill the 21:9 space. Sorry we skipped a step in our explanations.
You’ve discovered the issue already, but let’s walk through the math anyway because there is a much better solution available than what you’re doing now.
I assume your source video is 3840x2160. Shotcut by default is going to make the entire source video fit on the screen. It does not automatically scale or crop to fill the entire output space because discarding video data due to scaling should be a creator’s artistic decision, not a default decision. Since your source video is taller than your output aspect ratio, Shotcut is going to squish it down uniformly so the entire video can be seen on-screen, which leaves empty black space on the sides that will be visible even in Shotcut. You’re correct this is not a YouTube problem.
At this point, we need to stretch the source video to fill the width of the screen. If we maintain aspect ratio of the source while zooming/scaling/stretching it to fill the width, then the top and bottom will stretch/grow into the letterbox area and effectively be cropped out.
To find the exact scale factor to use, the shortened formula in this scenario is ZoomFactor = SourceHeight / OutputHeight. So 2160 / 1600 = 135% scale factor. Or 1440 / 1080 = 133.4% scale factor. At this point, the easiest thing to do in Shotcut is apply a Rotate and Scale filter, set the Scale parameter to 135%, and boom, done. You can then change the Y Offset if you want to pan the source video up or down to change what’s visible within the letterboxed area. This is substantially easier than fiddling with math in the Size and Position filter.
As you noticed, that is double effort. We can save you a lot of time here. If all your sources are 3840x2160, you can put a single Rotate and Scale filter on your V1 trackhead rather than the clip itself, and it will be applied to every clip on that track automatically. If you want to vertically shift an individual clip, you can add an additional Rotate and Scale filter to that specific clip. It doesn’t get easier than that.